Can I close and reopen a socket?

Question

I learned an example of usage of sockets. In this example a client sends a request to a server to open a socket and then the server (listening to a specific port) opens a socket and everything is fine, socket is "opened" from both sides (client and server).

But it is still not clear to me how flexible is this stuff. For example, is it possible for the client to close an opened (from both ends) socket and to reopen it again (under condition that the server keeps the socket opened).

Is it possible for the server to "know" that a socket was closed on the client side? Is it possible for the client to know that a socket was closed on the server side?

ADDED:

One more important thing to me. What happens if a application (no mater server or client) crashes, abnormally terminated, killed? Will it close all sockets opened on the side of the application?

ADDED 2:

What if an application on one side of the socket is switched off (killed, closed, terminated) and then it is switched on again (on the same IP address and the same port). Should we create a new socket between the two applications or we can use the old socket (created before the crash).

Answer 1

A socket can be used for a lot of things for which the answers to these questions would change, but I'll assume you're talking about TCP.

For example, is it possible for the client to close an opened (from both ends) socket and to reopen it again (under condition that the server keeps the socket opened).

No, because TCP will perform a goodbye and you can't pick up the connection from there again. You'd have to do the three-way handshake again, and that starts a brand new connection.

Is it possible for the server to "know" that a socket was closed on the client side? Is it possible for the client to know that a socket was closed on the server side?

Yes. TCP can send out a goodbye packet or one side can time out and it's entirely possible to detect these scenarios in both cases.

Answer 2

Is it possible for the server to "know" that a socket was closed on the client side?

When server tries to send some data to that client a correspondent exception will be thrown.

One more important thing to me. What happens if a application (no mater server or client) crashes, abnormally terminated, killed? Will it close all sockets opened on the side of the application?

Exceptions are created for handling these abnormal cases. If there is a black out and a client (or server) is turned off then other side will get an exception as soon as it try to interact with turned off side.

UPD:

What if an application on one side of the socket is switched off (killed, closed, terminated) and then it is switched on again (on the same IP address and the same port). Should we create a new socket between the two applications or we can use the old socket (created before the crash).

Create new socket.